In response to the development of various optical recording media in recent years, optical pickup devices that can record information on and reproduce information from two types of optical recording media have been commonly used. For example, devices that carry out recording and reproducing information with either a DVD (Digital Versatile Disk) or a CD (Compact Disk including CD-ROM, CD-R, CD-RW) have been used.
For these two types of optical recording media, the DVD uses visible light having a wavelength of approximately 657 nm for improved recording densities, while the CD is required to use near-infrared light having a wavelength of approximately 790 nm because there are some optical recording media that have no sensitivity to visible light. A single optical pickup device, known as a double wavelength pickup device, uses incident light of these two different wavelengths. The two optical recording media described above require different numerical apertures (NA) due to their different features. For example, the DVD is standardized to use light having a numerical aperture of about 0.6 through 0.65 and the CD is standardized to use light having a numerical aperture of about 0.45 through 0.52. Additionally, in these optical recording media, the substrate thickness, the thicknesses of the two types of recording disks, including the thicknesses of the protective layers or substrates made of polycarbonate (PC), are different. For example, the DVD may have a substrate thickness of 0.6 mm and the CD may have a substrate thickness of 1.2 mm.
As described above, because the substrate thickness of the optical recording medium is standardized and differs according to the type of optical recording medium, the amount of spherical aberration introduced by the substrate is different based on the different standardized thicknesses of the substrates of the different optical recording media. Consequently, for optimum focus of each of the light beams on the corresponding optical recording medium, it is necessary to optimize the amount of spherical aberration in each light beam at each wavelength for recording and reproducing. This makes it necessary to design the objective lens with different focusing effects according to the light beam and recording medium being used.
Additionally, in response to rapid increases of data capacity, the demand for an increase in the recording capacity of optical recording media has been strong. It is known that the recording capacity of an optical recording medium can be increased by using light of a shorter wavelength and by increasing the numerical aperture (NA) of an objective optical system. Concerning a shorter wavelength, the development of a semiconductor laser with a shorter wavelength using a GaN substrate (for example, a semiconductor laser that emits a laser beam of 408 nm wavelength) has advanced to the point where this wavelength is becoming practical. With the development of short wavelength semiconductor lasers, research and development of AODs (Advanced Optical Disks), also known as HD-DVDs, that provide an increased data storage capacity of approximately 20 GB with a single layer on one side of an optical disk by using light of shorter wavelength, has similarly progressed. As the AOD technical standard, the numerical aperture and disk thickness have been selected to be about the same as those of DVDs, as discussed previously, with the numerical aperture (NA) and disk substrate thickness for an AOD being set at 0.65 and 0.6 mm, respectively.
Furthermore, research and development of Blu-ray disk (BD) systems that use a shorter wavelength of disk illuminating light, similar to AOD systems, have also progressed. Moreover, the technically standardized values of numerical aperture and disk thickness for these systems are completely different from the corresponding DVD and CD values, with a numerical aperture (NA) of 0.85 and a disk substrate thickness of 0.1 mm being standard. Unless otherwise indicated, hereinafter AODs and Blu-ray disks collectively will be referred to as “AODs.”
Accordingly, the development of an optical pickup device that can be commonly used for three different types of optical recording media, such as AODs, DVDs and CDs, as described above, has been desired and objective lenses for mounting in such devices have already been proposed.
Among these objective lenses, objective lenses that include a diffractive surface on at least one objective lens surface are described in Japanese Laid-Open Patent Application 2001-195769. The objective lens described in this publication uses the diffracted light of a specified order from the diffractive surface corresponding to each of the optical recording media, such as the next generation of high density optical disks that may use, for example, light of a wavelength of 400 nm, as well as using diffracted light of a wavelength used with a DVD and diffracted light of a wavelength used with a CD. This results in improvement in spherical aberration, which differs generally due to differences in thicknesses of the substrates of the different optical recording media that make different contributions of spherical aberration, and in improvement in chromatic aberration, which tends to be large in an objective lens formed as a single lens element.
In addition, the objective lens of Japanese Laid-Open Patent Application 2001-195769 is constructed so that a divergent light beam is incident on the diffractive surface when information is recorded on or reproduced from a CD. However, when information is recorded on or reproduced from the next generation of high density optical disk (which may use light of 400 nm wavelength), or from a DVD, the design is such that a collimated light beam is incident on the diffractive optical element in order to control the deterioration in tracking accuracy that occurs in conjunction with the deterioration of optical performance off the optical axis.
A design where collimated light beams are incident on the objective optical system for all three light beams of different wavelengths, as described previously, is demanded in order to increase the degree of freedom in the arrangement of an optical system in an optical pickup device, which, in turn, may enable meeting the strong demand for a compact device.
In particular, concerning the light beam with the shortest wavelength of the three light beams described previously, if a convergent light beam enters the diffractive optical element, the diffraction efficiency deteriorates due to the tilted incidence of the light on the diffraction grooves of the diffractive optical surface, and the tracking stability is remarkably reduced. Additionally, in the case of forming the diffractive optical element on the surface of a lens, as mentioned previously, the processing accuracy becomes extremely deteriorated if the surface of the lens has a large curvature.
Furthermore, although the above-mentioned Japanese Laid-Open Patent Application 2001-195769 is an example where all three light beams of different wavelengths, as described previously, enter into an objective optical system as collimated light, the diffraction order of the light being used is not specifically considered, so it is difficult to achieve high diffraction efficiency in a balanced manner for all three light beams of different wavelengths.